Transcriptions of a set of class lectures delivered, during the six weeks of Lent, by St. Cyril of Jerusalem (then bishop and patriarch of Jerusalem) to a class of candidates planning on baptism and reception into the Church at Easter. Sometime during the fourth century.
At the time these lectures were being given, catechumens and candidates were not allowed to attend the entire Mass, learn many important teachings before being baptized, or disclose Church teachings to outsiders. This was called the "disciplina arcani". It was intended to protect the Church from persecution and its mysteries from profanation. So that's why it says at the end of the prologue lecture:
"You may lend these Catechetical Lectures for those who are to be enlightened to candidates for Baptism and to believers who are already baptized to read, but give them not at all either to catechumens, or to any others who are not Christians, as you shall answer to the Lord. And if you make a copy, write this in the beginning, as in the sight of the Lord."